1. Field of the Invention
The present invention relates in general to a control apparatus for a hybrid vehicle power transmitting system, and more particularly to techniques for promoting a warm-up operation of a hybrid vehicle power transmitting system including a differential mechanism operable to perform a differential function and all electric motor.
2. Discussion of Prior Art
There is known a hybrid vehicle power transmitting system including (a) a differential mechanism operable to distribute an output of an engine to a first electric motor and a power transmitting path between the differential mechanism and drive wheels, (b) a differential limiting device operable to limit or inhibit a differential function of the differential mechanism, and (c) a second electric motor connected to the power transmitting path. FIG. 1 of JP-2006-46386A shows an example of such a hybrid vehicle power transmitting system. A control apparatus for the hybrid vehicle power transmitting system disclosed in this publication is configured to operate the engine at a predetermined warm-up speed and place the differential mechanism in a differential state (in which the differential mechanism is operable to perform the differential function), when the control apparatus determines that it is necessary to warm up the engine or its exhaust system, for example, a catalyst device in the exhaust system.
According to the warm-up operation of the hybrid vehicle power transmitting system performed under the control of the control apparatus disclosed in the above-identified publication, the operating speed of the engine is not bound or influenced by a during speed of the hybrid vehicle, owing to the differential function of the differential mechanism, so that the warm-up operation of the engine and its exhaust system can be effectively promoted. However, not only the engine and its exhaust system but also other components of the power transmitting system such as the differential mechanism are desirably subjected to a warm-up operation. For instance, it is desirable to warm up the differential mechanism, for raising the temperature of a lubricant for the differential mechanism so that the lubricant has a suitable degree of viscosity. The promotion of the warm-up operation of the hybrid vehicle power transmitting system as a whole permits an improvement of fuel economy of the hybrid vehicle. However, the control apparatus disclosed in the above-identified publication does not necessarily assure sufficient promotion of the warm-up operation of the various mechanisms and devices such as the electric motors of the hybrid vehicle power transmitting system.